24 



OF WOOD IN GENERAL 



oak, we find, with the same general exogenous arrangement of 

 pith, bark, heart-wood, sap-wood, arid annual rings, considerably 

 greater complexity in the variety and grouping of the elements 

 of which the tissues are built up (Fig. 18). The pith presents 



Fig. 19.— Transverse section of Oak, pliotogi-aphed direct from nature. 



considerable variety among broad-leaved trees, so as to be used 

 to some extent in discriminating woods seen in complete cross- 

 sections. Thus in its proportion to the area of the wood in cross- 

 section it may vary from equahty, i.e. being as wide as the xylem. 



as in three-year-old shoots of Elder, 



^^ 2 5 5 



as in shoots of the 



Fig. 20.-Part of transverse section through a branch of Cork Oak (Onerous Suber), 

 /■f^^^^T?^^' ^^fi^^' LeMaoutand Decaisne, from The Elements of Botany ;hy permission 

 of Mr. Francis Darwin and the Syndicate of the Cambridge University Press ) 



M, pith ; PC, phloem and cortex ; S, cork ; 1, primary pith-ray, running from pith 

 to cortex ; 2, 3, and 4, secondary pith-rays formed in successive years. 



Cork-Elm of the same age. In outline it may be pentangular or 

 hexagonal, as in Oak, Spanish Chestnut, Black Poplar, or White 

 Willow ; triangular, as in Birch, Beech, and conspicuously in Alder ; 

 ovoid, as Linden, Plane, Holly, Ash, and Maples ; or nearly circular! 



